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A weedy potato field in Beaconsfield, N.B. Photo: Dave Bell

Loosing Linuron

In the not-too-far future, Canadian potato producers may have to look for other options to replace this valuable tool for controlling weeds and managing herbicide resistance.

Linuron is a staple ingredient in herbicides used for decades to help Canadian growers control broadleaf and grassy weeds prior to planting potatoes each growing season. However, changes are coming that will affect growers who rely on this pre-plant burndown product to control persistent weeds.

Linuron is an active ingredient in some Group 7 herbicide products that growers have been using effectively, not just to control weeds and to also fight against herbicide resistance, which has been a growing problem for potato producers across Canada in the past decade.

Because it provides a different mode of action than other herbicides that contain glyphosate, triazines and other active ingredients, linuron is used by many Canadian growers to help prevent and break the cycle of herbicide resistant weeds.

Linuron is used to control both annual and perennial broadleaf weeds as well as grassy weeds. In Canada, potato growers apply it as a pre-emergence treatment.

According to New Brunswick-based agronomist Dave Bell, linuron is long-lasting and works very well in row-forming, one-pass hilling for growers. “Being able to use different modes of action in crop rotations discourages resistance developing to herbicides. Linuron has been an effective crop rotation tool for producers in the province,” he says.

Mackenzie Lespérance, acting weed management horticulture program lead with Ontario Ministry of Agriculture, Food and Rural Affairs, explains that for growers with tough to control weeds such as pigsweed and lamb’s quarter, linuron has been an effective tool. “Banning linuron will simply offer potato growers fewer choices to control early weeds and overall weed suppression,” she says.

Proposed Phase-out

After a re-evaluation of linuron in 2012, Health Canada’s Pest Management Agency proposed that the sale and use of all linuron products should be phased out in Canada due to health and environmental concerns. A Health Canada webpage pertaining to linuron states that “an evaluation of available scientific information found that, under the current conditions of use, the human health and environmental risks estimated for linuron do not meet current scientific standards.”

Health Canada media relations officer André Gagnon says a special review of linuron is slated to be completed for 2017-2018, and indicated there likely wouldn’t be any changes regarding its use until then. “Considering the work involved and the amount of detailed specific information to be reviewed, any changes to labels, if required, would be made upon the completion of the review,” he says.

Gagnon also notes that proposed timelines may change in response to emerging issues that require priority action, and at any point during the special review of linuron, “should evidence become available demonstrating reasonable grounds that it endangers human health or the environment, the PMRA will cancel or amend the pest control product’s registration.”

 Serious Consequences

Plant scientist Clarence Swanton of the University of Guelph says if linuron is taken off the market, it will have a definite impact on the potato industry. “It’s significant,” states Swanton. “There are other products available for use but linuron has been a very effective treatment for potatoes because it has a broad range in controlling broadleaf weeds.”

Bell agrees the consequences of taking linuron off the market would be serious. “Traditionally, growers have used metribuzin and linuron as the two main products to rotate between to keep product efficacy,” he explains. “While there are some other alternative products available the loss of linuron could cause additional problems down the road.”

With limited options for alternative herbicide treatments, one industry concern is that an increased reliance on metribuzin by potato growers and the horticulture industry as a whole would increase the incidence of herbicide-resistant weeds.

“There already is resistance in some locations to metribuzin which not only limits its effectiveness but also limits growers’ options for an alternative in the marketplace,” explains Swanton.

“Potato farming is pretty intensive on the soil profile and farmers are aware of the need for effective options to fight resistant weeds,” says Bell.

“We are already witnessing … glyphosate resistance in certain geographic regions,” he adds. “It’s an ongoing and increasing concern for all growers.”

Swanton says having fewer herbicides available could harm the potato industry over the long term.

“With only a handful of products to choose, from the end result is that it will only be a matter of time before herbicide resistance will show up here with those [other] products as well,” states Swanton. “This [would be] a significant hit for weed management in potatoes with all product uses cancelled for linuron.”

Managing Resistance

Lespérance notes that one major key to avoiding herbicide resistance is by using different modes of action each year as well as throughout the growing season.

“Linuron has been cost-effective, controls early emerging annuals and offers a broad spectrum of annual residual control,” she explains. “Removing a herbicide group from growers tool kits will cause greater dependence on other modes of action, making resistance management more important than ever.”

While there are currently other alternatives in the marketplace, Bell says that the newer products tend to be higher priced.

He adds that if linuron becomes unavailable, properly managed crop rotations will be even more important in controlling herbicide resistance. “It’s important to build organic matter within the soil but also offer the use of alternative herbicide products through crop rotation,” he says.

“Corn is a common rotation crop to help decrease herbicide resistance and promote soil health,” Bell says, adding that forages and soybeans are other beneficial rotational crops that add nutrients and also promote soil-building activity.

Lespérance says the best approach for producers is to be pro-active in terms of weed management and weed suppression. “If you lose a herbicide you are losing your best horse in the race but there are proactive options for farmers to implement on their farms to help with weed management,” she adds.

These options include:

  1. Use multiple modes of action – in-year and year to year
  2. Maintain proper crop rotations
  3. Consider seedbed preparation
  4. Use cover crops for inter-competition
  5. In-crop cultivation

Lespérance notes that growers are unlikely to avoid herbicide resistance altogether but through a proactive approach they can limit the severity of resistance on their farms.

Next Steps

According to Lespérance, ongoing herbicide resistance research is being conducted in main growing regions in Ontario to help create new options for growers.

Bell and Swanton agree that as farmers face the loss of an important herbicide treatment option, there’s a great need within the Canadian potato industry for research into possible product replacements for linuron.

“There really is a need for proactiveness within the industry to provide new and viable options for growers,” explains Bell.

“Stakeholders need to address the limited list of products already available to potato growers and step up to the plate to become a more pro-active influence for all vegetable and horticultural growers,” says Swanton.

 

WHERE ON THE WEB:

For more information concerning the status of Health Canada’s PMRA re-evaluation process of linuron visit:

hc-sc.gc.ca/cps-spc/pest/part/consultations/_prvd2012-02/prvd2012-02-eng.php#a3.

 

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